Efforts to whiten teeth have a long history, thought to date back to the Ancient Egyptians. Modern science has provided a detailed understanding of the factors that contribute to tooth color, which has enabled improved products and methods for whitening. The normal shade of teeth is determined by the natural off-white tints of the enamel and the dentin beneath. Extrinsic and intrinsic staining also contribute to tooth color.
Extrinsic staining refers to surface stains, such as those caused by tea, coffee, red wine, and other foods rich in polyphones. Extrinsic staining primarily occurs as a result of charged surface interactions between the positively-charged food molecules and the negatively-charged tooth pellicle, the protein film on the tooth surface that is derived from salivary proteins. Extrinsic stains are removed through the use of surfactants and/or abrasives, which cause their physical removal from the tooth surface.
Intrinsic staining refers to stains that exist below enamel surface, or that penetrate below enamel surface. Intrinsic staining can happen when food molecules seep into enamel flaws and cracks, or, in some cases, between enamel rods. Intrinsic discoloration can also occur following a change to the structural composition or thickness of the dental hard tissues. Certain metabolic diseases and tooth trauma can also cause intrinsic staining. Tetracyline also causes intrinsic staining.
Removal of intrinsic staining is more difficult and time consuming than removal of extrinsic staining. Intrinsic stain removal can be achieved by a variety of methods including use of peroxides or peroxide analogs, with or without chemical, light or heat activation, to bleach the stains. This method oxidizes organic compounds within the enamel and dentin, thereby changing colored materials to non-colored materials; it does not remove the stain itself. Acids and dehydration methods, which lead to opacification of enamel to obscure the subsurface stains, are also used to remove or mask intrinsic staining.
Tooth whitening products are available over-the-counter and as professional services in a dentist's office. Over-the-counter products typically contain carbamide peroxide or hydrogen peroxide as the bleaching agent. These products have concentrations of up to 21% carbamide peroxide (equivalent to 7% hydrogen peroxide) or as much as 10% hydrogen peroxide. They also contain carbomers (for thickening and control) and acidifiers (for peroxide stabilization in aqueous solution), or alternatively have an anhydrous glycerin base. In-office treatments generally use hydrogen peroxide as the oxidizer, at concentrations of 15% or more, and typically in the 25 to 35% range. At these high concentrations, rubber dams, or liquid dams with proper suction, must be used to prevent gingival irritation and ingestion. Additionally, due to their high strengths, products for professional treatment require more thickener and more acidification to make them stable, compared to home-use products. Furthermore, professional chair-side formulas have a secondary and often even tertiary and quaternary activators to stimulate a more rapid result. These activators take the form of pH modifiers, light sources, and heat sources.
Tooth whitening products, both over-the counter and particularly professional treatments, have several unpleasant side effects, including tooth sensitivity, soft tissue irritation and tooth surface changes.
Transient tooth sensitivity is the most common side effect reported. Hydrogen peroxide and carbamide peroxide have not been found to induce pathological pulpal changes in testing, although 10% carbamide peroxide has been reported as causing mild, reversible histological changes. It is believed that the hypersensitivity associated with whitening is caused by dehydration, due to the acidified and thickened, substantially anhydrous, hydrophillic gels used in the peroxide formulations and that are held against the teeth. Dehydration results in a negative osmotic pressure and in odontoblastic processes being drawn into the dentinal tubules. Other factors that may contribute to dehydration include whitening lights used in in-office treatments. While sensitivity may be transient, it is a very undesirable side effect.
Oral mucosa irritation is the second most-common side effect reported. Systems using higher concentrations of hydrogen peroxide or carbamide peroxide result in more gingival irritation than lower concentration formulations. While peroxide is regarded as safe at low concentrations, peroxide has the potential to induce cell changes at high concentrations over an extended period of time.
Tooth surface changes have been observed for hydrogen peroxide and carbamide peroxide in in-vitro testing. Although recent reports on this issue have conflicting results, this aspect of tooth whitening products currently available remains a point of concern. In addition, tooth whitening formulations are usually acidic; acids can decalcify and etch teeth, causing a temporary opacification of underlying discolorations. These side effects often result in the need for remineralization therapies in connection with tooth whitening treatment, particularly those using professional products.
Another problem with current tooth whitening formulations is rebound. Rebound is the phenomenon wherein stains re-appear after a relatively short period of time after tooth whitening. The time that elapses post-treatment before this occurs varies from a few days to weeks, while other studies claim up to 47 months before any rebound effect occurs (Leonard et al., 2001, J. Esthet. Restor. Dent. 13(6): 357-369). One study found a rebound in 40% of patients at six months with use of concentrations ranging from 16%-18% carbamide peroxide (Brunton et al., 2004, Oper. Dent. 29(6): 623-626).
Chlorine dioxide (ClO2) has been suggested as an alternative to peroxide based bleaching agents for tooth whitening applications. Chlorine dioxide (ClO2) is well known as a disinfectant as well as a strong oxidizing agent. The bactericidal, algaecidal, fungicidal, bleaching, and deodorizing properties of chlorine dioxide are also well known.
Chlorine dioxide (ClO2) is a neutral compound of chlorine in the +IV oxidation state. It disinfects by oxidation; however, it does not chlorinate. It is a relatively small, volatile, and highly energetic molecule, and a free radical even in dilute aqueous solutions. Chlorine dioxide functions as a highly selective oxidant due to its one-electron transfer mechanism in which it is reduced to chlorite (ClO2−). The pKa for the chlorite ion/chlorous acid equilibrium, is extremely low at pH 1.8. This is remarkably different from the hypochlorous acid/hypochlorite base ion pair equilibrium found near neutrality, and indicates that the chlorite ion will exist as the dominant species in drinking water.
One of the most important physical properties of chlorine dioxide is its high solubility in water, particularly in chilled water. In contrast to the hydrolysis of chlorine gas in water, chlorine dioxide in water does not hydrolyze to any appreciable extent, but remains in solution as a dissolved gas.
The traditional method for preparing chlorine dioxide involves reacting sodium chlorite with gaseous chlorine (Cl2(g)), hypochlorous acid (HOCl), or hydrochloric acid (HCl). The reactions are:2NaClO2+Cl2(g)=2ClO2(g)+2NaCl  [1a]2NaClO2+HOCl=2ClO2(g)+NaCl+NaOH  [1b]5NaClO2+4HCl=4ClO2(g)+5NaCl+2H2O  [1c]Reactions [1a] and [1b] proceed at much greater rates in acidic medium, so substantially all traditional chlorine dioxide generation chemistry results in an acidic product solution having a pH below 3.5. Also, because the kinetics of chlorine dioxide formation are high order in chlorite anion concentration, chlorine dioxide generation is generally done at high concentration (>1000 ppm), which must be diluted to the use concentration for application.
Chlorine dioxide may also be prepared from chlorate anion by either acidification or a combination of acidification and reduction. Examples of such reactions include:2NaClO3+4HCl→2ClO2+Cl2+2H2O+2NaCl  [2a]2HClO3+H2C2O4→2ClO2+2CO2+2H2O  [2b]2NaClO3+H2SO4+SO2→2ClO2+2NaHSO4  [2c]At ambient conditions, all reactions require strongly acidic conditions; most commonly in the range of 7-9 N. Heating of the reagents to higher temperature and continuous removal of chlorine dioxide from the product solution can reduce the acidity needed to less than 1 N.
A method of preparing chlorine dioxide in situ uses a solution referred to as “stabilized chlorine dioxide.” Stabilized chlorine dioxide solutions contain little or no chlorine dioxide, but rather, consist substantially of sodium chlorite at neutral or slightly alkaline pH. Addition of an acid to the sodium chlorite solution activates the sodium chlorite, and chlorine dioxide is generated in situ in the solution. The resulting solution is acidic. Typically, the extent of sodium chlorite conversion to chlorine dioxide is low and a substantial quantity of sodium chlorite remains in the solution.
U.S. Pat. No. 6,582,682 discloses an oral care composition comprising “stabilized chlorine dioxide” that, upon exposure to the mildly acidic pH in the oral cavity, produces chlorine dioxide.
U.S. Pat. No. 6,479,037 discloses preparing a chlorine dioxide composition for tooth whitening wherein the composition is prepared by combining a chlorine dioxide precursor (CDP) portion with an acidulant (ACD) portion. The CDP portion is a solution of metal chlorite at a pH greater than 7. The ACD is acidic, preferably having a pH of 3.0 to 4.5. The CDP is applied to the tooth surface. The ACD is then applied over the CDP to activate the metal chlorite and produce chlorine dioxide. The pH at the contact interface is preferably less than 6 and, most preferably, in the range of about 3.0 to 4.5. Thus, the resulting chlorine dioxide composition on the tooth surface is acidic. Additionally, this method exposes the oral mucosa to possible contact with a strongly highly acidic reagent (ACD).
There is a need in the art for tooth whitening compositions and methods with reduced side effects.